Ketonethiosemicarbazones: structure-activity relationships for their melanogenesis inhibition

A series of 2-(1-phenylalkylidene)hydrazinecarbothioamides 2, 2-(1-phenylalkyl)hydrazinecarbothioamides 3, 2-(3,4-dihydronaphthalen-1(2H)-ylidene)hydrazinecarbothioamide (4), and 2-(1-(thiophen-2-yl)ethylidene)hydrazinecarbothioamide (5) were synthesized for their melanogenesis inhibition in melanoma B16 cells. The SAR of these ketonethiosemicarbazones revealed that the benzylidene hydrogen in aldehydethiosemicarbazones 1 can be replaced by hydrophobic moiety and substitutions with alkyl group for the terminal amino hydrogen of ketonethiosemicarbazones improved the activity appreciably. In addition, the double bond in thiosemicarbazones is an important factor for the increment of hydrophobicity. Thus hydrophobic ketonethiosemicarbazones are excellent inhibitors of melanogenesis like aldehydethiosemicarbazones.     

Melanogenesis‐Inhibitory Activities of Isomeric C‐seco Limonoids and Deesterified Limonoids

Treatment of eight C‐seco limonoids including six of salannin‐type, 1 – 6 , and two of nimbin‐type, 7 and 8 , with a combination of BF₃ · Et₂O and iodide ion yielded the isomeric C‐seco derivatives, i.e., six isosalannins, 1a – 6a , and two isonimbins, 7a and 8a , respectively. Ohchinin ( 1 ) was further subjected to LiAlH₄ reduction which yielded a deesterified trihydroxy limonoid, nimbidinol ( 9 ). In addition, ten limonoids including seven of azadirone‐type, 10 – 16 , and three of gedunin‐type, 17 – 19 , all of which possess no ester functionality in the molecule, were obtained from the neutral fraction of Azadirachta indica seed extract after alkaline hydrolysis. Among the above, twelve compounds, i.e., 1a – 4a , 6a , 9 , 13 – 16 , 18 , and 19 , were new compounds, and their structures were elucidated on the basis of extensive spectroscopic analysis and comparison with literature data. Upon evaluation of all these limonoids for their inhibitory activities against melanogenesis in B16 melanoma cells induced with α‐melanocyte‐stimulating hormone (α‐MSH), five structurally modified limonoids, 3‐deacetyl‐28‐oxosalannin ( 6a ), 9 , 17‐epi‐17‐hydroxynimbocinol ( 14 ), 17‐epi‐17‐hydroxy‐15‐methoxynimbocinol ( 15 ), and 7‐deacetyl‐17‐epinimolicinol ( 18 ), in addition to a natural limonoid, 1 , exhibited potent inhibitory activities with 26 – 66% reduction of melanin content at 100 μm concentration with almost no or low toxicity to the B16 melanoma cells (70 – 99% cell viability at 100 μm ).     

Melanogenesis inhibitory activity of a 7-O-9′-linked neolignan from Alpinia galanga fruit

An aqueous acetone extract from the fruit of Alpinia galanga (Zingiberaceae) demonstrated inhibitory effects on melanogenesis in theophylline-stimulated murine B16 melanoma 4A5 cells (IC₅₀ = 7.3 μg/mL). Through bioassay-guided separation of the extract, a new 7-O-9′-linked neolignan, named galanganol D diacetate (1), was isolated along with 16 known compounds including 14 phenylpropanoids (2–15). The structure of 1, including its absolute stereochemistry in the C-7 position, was elucidated by means of extensive NMR analysis and total synthesis. Among the isolates, 1 (IC₅₀ = 2.5 μM), 1′S-1′-acetoxychavicol acetate (2, 5.0 μM), and 1′S-1′-acetoxyeugenol acetate (3, 5.6 μM) exhibited a relatively potent inhibitory effect without notable cytotoxicity at effective concentrations. The following structural requirements were suggested to enhance the inhibitory activity of phenylpropanoids on melanogenesis: (i) compounds with 4-acetoxy group exhibit higher activity than those with 4-hydroxy group; (ii) 3-methoxy group dose not affect the activity; (iii) acetylation of the 1′-hydroxy moiety enhances the activity; and (iv) phenylpropanoid dimers with the 7-O-9′-linked neolignan skeleton exhibited higher activity than those with the corresponding monomer. Their respective enantiomers [1′ (IC₅₀ = 1.9 μM) and 2′ (4.5 μM)] and racemic mixtures [(±)-1 (2.2 μM) and (±)-2 (4.4 μM)] were found to exhibit melanogenesis inhibitory activities equivalent to those of the naturally occurring optical active compounds (1 and 2). Furthermore, the active compounds 1–3 inhibited tyrosinase, tyrosine-related protein (TRP)-1, and TRP-2 mRNA expressions, which could be the mechanism of melanogenesis inhibitory activity.     

Vitiligo,Psoralen, and Melanogenesis: Some Observations and Understanding

Since the etiology of vitiligo is still unknown, we searched for some abnormal biochemical parameters, if any, in subjects with vitiligo. Higher urinary excretion of indole metabolites in vitiliginous patients have been noted, in association with higher dioxygenase, superoxide dismutase, and tyrosine aminotransferase activity in their serum. Similar results have also been found in an animal model, Bufo melanostictus, during induced tyrosinase inhibition. Treatment with psoralen can reverse the parameters, except tyrosine aminotransferase, to a normal level. Although psoralens are not the magic bullet for the therapy of vitiligo, they are still being used as a chemotherapeutic agent against vitiligo on a major scale to date. Tryptophan was found to participate in the pathway of melanogenesis, as a precursor as well as a positive regulator of tyrosinase. Its behavior in this regard is much more similar to the conventional substrates tyrosine and dopa (dihydroxyphenylalanine). In consideration of combined participation of tyrosine and tryptophan in the synthesis of melanin and its breakdown, the possible influence of different enzymatic reactions, like mono-oxygenase, dioxygenase, and deamination, has been suggested.     

Bicyclic Monoterpene Diols Induce Differentiation of S91 Melanoma and PC 12 Pheochromocytoma Cells by a Cyclic Guanosine-Monophosphate-Dependent Pathway

Previously, we showed that 5-norbornene-2,2-dimethanol (5-NBene-2,2-DM) is an effective inducer of melanogenesis in cultured cells and guineapig skin [Brown et al. (1998) J. Invest. Dermatol., 110:428-437]. This study shows that 2,3-cis/exo-pinanediol (2,3-cs/ex-PinD) is a more effective inducer of melanogenesis than 5-NBene-2,2-DM in S91 mouse melanoma cells. Furthermore, 2,3-cs/ex-PinD appears to penetrate guinea-pig skin better than 5-NBene-2,2-DM and to induce higher levels of pigmentation. Both 5-NBene-2,2-DM and 2,3-cs/ex-PinD induce synthesis of nitric oxide (NO) in S91 cells, and the melanogenic activity of both compounds is reduced by inhibitors of the NO/cyclic guanosine monophosphate (cGMP)/protein kinase(PK) G signaling pathway, but not by inhibitors of the PKC or PKA pathways. Thus, these bicyclic monoterpene diols appear to induce melanogenesis by the same pathway in S91 cells as that shown previously for ultraviolet radiation in melanocytes (Romero-Graillet et al. (1996) J. Biol. Chem., 271:28052-28056). These compounds also induce NO synthesis, neurite outgrowth, and tyrosine hydroxylase activity in PC 12 pheochromocytoma cells. Neurite outgrowth in PC 12 cells is blocked by the guanylate cyclase inhibitor, LY83583 (6-anilino-2,8-quinolinequinone), indicating that, similar to S91 cells, the induction of morphological differentiation of PC12 cells by bicyclic monoterpene diols is regulated by a cGMP-dependent pathway.     

Assembly,Target‐Signaling and Intracellular Transport of Tyrosinase Gene Family Proteins in the Initial Stage of Melanosome Biogenesis

Assembly, target‐signaling and transport of tyrosinase gene family proteins at the initial stage of melanosome biogenesis are reviewed based on our own discoveries. Melanosome biogenesis involves four stages of maturation with distinct morphological and biochemical characteristics that reflect distinct processes of the biosynthesis of structural and enzymatic proteins, subsequent structural organization and melanin deposition occurring in these particular cellular compartments. The melanosomes share many common biological properties with the lysosomes. The stage I melanosomes appear to be linked to the late endosomes. Most of melanosomal proteins are glycoproteins that should be folded or assembled correctly in the ER through interaction with calnexin, a chaperone associated with melanogenesis. These melanosomal glycoproteins are then accumulated in the trans Golgi network (TGN) and transported to the melanosomal compartment. During the formation of transport vesicles, coat proteins assemble on the cytoplasmic face of TGN to select their cargos by interacting directly or indirectly with melanosomal glycoproteins to be transported. Adapter protein‐3 (AP‐3) is important for intracellular transport of tyrosinase gene family proteins from TGN to melanosomes. Tyrosinase gene family proteins possess a di‐leucine motif in their cytoplasmic tail, to which AP‐3 appears to bind. Thus, the initial cascade of melanosome biogenesis is regulated by several factors including: 1) glycosylation of tyrosinase gene family proteins and their correct folding and assembly within ER and Golgi, and 2) supply of specific signals necessary for intracellular transport of these glycoproteins by vesicles from Golgi to melanosomes.     

Mutational Analysis of the Modulation of Tyrosinase by Tyrosinase‐Related Proteins 1 and 2 In Vitro

The albino (tyrosinase, Tyr^c), brown (tyrosinase‐related protein 1, Tyrp1^b) and slaty (tyrosinase‐related protein 2, tyrp2^slt) loci are all involved in the regulation of melanogenesis. Phenotypes of inbred mice mutant at two or more of these loci are not always explicable by simple summation of the established or suspected catalytic functions of the gene products. These phenotypes suggest that relationships among the proteins extend beyond the obvious fact that they catalyze different steps in the same melanogenic pathway, and that they may also interact intimately in such a way that a mutation in one impacts the function of the other(s). Previous studies have attributed catalytic activities to each member of this trio; however, it has been difficult to study the proteins individually, either in vivo or in tissues or cells. Therefore, we undertook to transfect the genes, in revealing combinations, into COS‐7 cells (which have no melanogenic apparatus of their own) to clarify the interacting functions of their encoded proteins. Specifically, we attempted to evaluate the effects of Tyrp1 and Tyrp2 proteins on tyrosinase protein. We report evidence that Tyrp1 stabilizes tyrosinase, confirming previous observations, and, in addition, demonstrate that Tyrp1 decreases tyrosinase activity. By contrast, Tyrp2 increases tyrosinase activity by stabilizing the protein. We conclude that both Tyrp1 and Tyrp2, in addition to other catalytic functions they may possess, act together to modulate tyrosinase activity.     

Composition of Mammalian Eumelanins: Analyses of DHICA-Derived Units in Pigments From Hair and Melanoma Cells

The proportions in which two eumelanin monomers, namely 5,6-dihydroxyindole-2-carboxylic acid (DHICA) and 5,6-dihydroxyindole (DHI), compose the eumelanin polymer are believed to determine properties of the pigment including its color. These proportions are, however, not well elucidated for naturally occurring eumelanins, largely because of methodological difficulties. In this study we estimate the content of DHICA-derived units in mammalian eumelanins using a combination of two analytical techniques: 1) quantitation of DHICA-derived eumelanin by measuring the yield of pyrrole-2,3,5-tricarboxylic acid (PTCA index) and 2) spectrophotometrical quantitation of total (DHI + DHICA) eumelanin at 350 nm (A₃₅₀ index). The ratio of PTCA/A₃₅₀ measured for melanins synthesized from DHI and DHICA mixed in various molar proportions correlates well with the content of DHICA in synthetic polymers. Using this relationship as a standard curve we estimated the proportion of DHICA-derived units in mammalian eumelanins from hair and melanoma cells and found it to be much higher in rodent pigments (58.8%-98.3%; two species, mouse and hamster were examined) as compared to human eumelanins (19.2%-41.8%; one Caucasian and one Oriental individual were examined). No relationship between proportion of DHICA-derived units in eumelanin and hair color is found. The latter seems to be determined predominantly by the ratio of pheo- to eumelanin synthesis.     

On the Melanization of the Rat's Eye

Electron-microscopic study of the size of the melanosomes, the mean percentage of melanosomal profile area (MPMA) of the cells, and the duration of melanogenesis in the pigmented layers of the rat's eye (inbred strain BDE/Han) revealed the following: 1) The melanosomes in the cells of the retina vary in size and shape in different locations of the eye. The MPMA of the cells also differs. Only in the two layers of the iris epithelium do the minor diameters of the melanosomes not differ significantly from each other, but the MPMA of the cells is different. The pigmented outer layer of the ciliary epithelium stands out on account of its especially large, round melanosomes. 2) The melanosomes of the uveal melanocytes are uniformly small but exhibit the largest MPMA. 3) Only in the pigment epithelium of the fundus does melanogenesis cease in the fifth week of life. As a result the MPMA decreases. In the other areas of the pigmented epithelium and the uvea tyrosinase activity and premelanosomes are present from the new-born to the adult animal. These signs indicate continued melanogenesis. 4) Compound melanosomes are present in all pigmented locations of the eye. Giant melanosomes occur regularly only in the outer layer of the retina.     

β-Lactoglobulin Suppresses Melanogenesis in Cultured Human Melanocytes

The effects of whey proteins from bovine milk on melanogenesis in cultured human melanocytes were examined. Among the major protein components of milk whey including β-lactoglobulin (BLG), α-lactalbumin, serum albumin, and IgG, only BLG exhibited the depigmenting effect at a concentration of 1 mg/ml. Also, BLG suppressed the activity of tyrosinase in these cells. Retinol, to which BLG is known to bind, slightly increased the pigmentation of the cells at concentrations in the range of 1–100 nM, and retinoic acid, a metabolite of retinol, exhibited a strong pigmentation-promoting effect within the same concentration range. Treatment of the cells with 1 mg/ml BLG completely abrogated the pigmentation induced by these A vitamins. These results demonstrate a novel biological activity of BLG and suggest that this activity is dependent on its ability to bind retinol.